Particle size dependent sinterability and magnetic properties of recycled HDDR Nd–Fe–B powders consolidated with spark plasma sintering

Spark Plasma Sintering as an Effective Texturing Tool for Reprocessing Recycled HDDR Nd-Fe-B Magnets with Lossless Coercivity

“…It confirms that these conditions are suitable for attaining fully dense and high coercivity submicron sized Nd 2 Fe 14 B grains by restricting their exaggerated grain growth and avoiding unnecessary transformation of the Nd-rich phase to Nd-oxides at elevated temperatures, as observed in the inset of Figure 2d. The backscattered electron imaging (presented in Figure 2d) provides the contrast to identify different phases in the complex microstructure of HDDR Nd-Fe-B system with grey matrix phase of submicron sized body centered tetragonal (bct) Nd 2 Fe 14 B grains, Nd 2 O 3 type oxides (cubic or hcp), and the bright intergranular Nd-rich phase (fcc-Nd or NdO X type), which matches the previous reports [7,8]. The dark spots in the vicinity of matrix grains and triple pockets are pores.…”

“…Therefore, the powder was not milled to prevent additional oxide phases which degrade sinterability, density, remanence (reduction in ferromagnetic phases) [8], and subsequently the texture from hot deformation [1,19]. The hot pressing conditions were kept similar to the previously reported SPS work [8] in order to achieve high coercivity and the optimal microstructure, i.e., at 750 • C for 1 min, 100 MPa uniaxial pressure and the post-SPS thermal treatment was also done for 1 h at 750 • C (above ternary transition temperature of 665 • C) [7]. These annealed hot-pressed magnets had H Ci ≥ 1120 kA/m and B r ≥ 0.79 T, with M r /M S ≈ 0.5 indicating isotropic nature of the bulk magnets prior to hot deformation.…”

“…In a similar sense, the Laves phases are not anticipated to have deleterious effects during the hot deformation. The detailed phase analysis of the recycled HDDR powder is also presented in the preceding studies on explaining the oxygen content relationship to the particle size, microstructural changes, and sinterability [8].…”

“…Due to this reason, either the bonded magnets are their primary application [5] or the rapid processing methodologies are applied, e.g., spark plasma sintering, hot deformation, and die-upsetting [1,6,7]. Previously we have demonstrated the control in SPS parameters resulted in high coercivity (H Ci ≈ 1200 kA/m) of the recycled HDDR Nd-Fe-B-type isotropic magnets [7] and that the particle size has a critical significance on the sinterability and oxygen uptake of the recycled powder [8]. These microstructure-magnetic properties relationships were derived only for the isotropic magnets (M r /M S ≤ 0.5) processed by SPS, whereas the starting recycled HDDR Nd-Fe-B powder contains anisotropic powder particles.…”

“…In order to enhance the remanence and the energy product, the hot deformation methodology is necessary [9] but unlike so far, the hot forging technique has not been adopted for the recycled HDDR Nd-Fe-B system. Besides oxygen content also plays an important role in the magnet reprocessing by hot deformation and since the recycled materials contain approximately two folds or higher oxygen content than the commercial grade materials, the processing route needs to be validated [8].…”